1. Field of the Invention
The present invention relates to electric parts for shielding noise generated on a signal line in various electronic equipment such as a computer.
2. Background of the Invention
Electromagnetic noise escaping from the cabinet and signal lines of electronic equipment causes a problem. Since these noises can be close to the control signal level, they can cause electromagnetic interference (EMI). This can lead to malfunctions or partial deletion of magnetic information recordings. A shielding structure to absorb noises is known in which a tubular ferrite core is mounted around a signal line. FIG. 5 is an example of such a conventional shielding structure.
In FIG. 5, the shielding parts 62 include a single magnetic element 63 such as a ferrite core and a plastic package 64 mounted surrounding the outer surface of the magnetic element 63. Because this shielding structure includes the single magnetic element 63 inside the package 64, the frequency band of noise to be absorbed is limited to a relatively narrow frequency band that can be absorbed by the single magnetic element 63. Therefore, other methods have been utilized to expand the frequency band of noise to be absorbed so that the required attenuation can be obtained. For example, a plurality of shielding parts 62 can be mounted on the signal line 61. Identical shielding parts 62 may be used or parts having different magnetic characteristics of magnetic substance 63 can be used to make up this plurality of shielding parts 62.
FIG. 6 shows the relationship between the frequency and noise attenuation when a plurality of the identical shielding parts 62 are mounted on the signal line 61. FIG. 7 shows the same relationship when a plurality of the shielding parts are used, in which different characteristics of magnetic element 63 are used. FIG. 6 was prepared based upon measured values. The curve represented by (c) shows the characteristics when only one shielding part 62 is utilized. The curve (d) shows the result of using two shielding parts 62 with the same magnetic element 63. On the other hand, the curve (e) in FIG. 7, which was also based upon measured values, shows the relationship between the frequency and noise attenuation when two shielding parts 62 are used. In this curve, each part 62 has a single magnetic element 63 inside the package 64, but with differing magnetic characteristics. As shown in FIG. 6, the effect of noise absorption has been dramatically improved but only in a certain narrow frequency band. Frequencies outside of this certain band are not effectively attenuated. Therefore, the enhanced attenuation is not obtained over a broader frequency band.
On the other hand, as shown in FIG. 7, enhanced attenuation can be obtained over a relatively wide frequency band when the two shielding parts 62 with a different characteristic of a magnetic element 63 are used. However, the above conventional noise shielding structure requires a plurality of shielding parts be independently mounted to make the frequency band for noise absorption wider. Due to the increase of assembling steps, manufacturing cost becomes higher resulting in an increased product price (particularly if the parts are installed manually). Further, as a plurality of the noise shielding parts 62 are independently assembled, precise positioning during an assembly cannot be assured. This invites inconsistency in the quality of the product.